An expandable-type plunger of the type to which this invention relates is useful in the production of containers. Initially it is moved downward to insert an internal closure into the upper end portion of a tubular container sleeve. Such an end closure has a flat lid portion which extends across the sleeve and has an upwardly projecting rim portion which surrounds its lid portion and which overlies the inner surface of the sleeve all around the same. When the end closure has been brought into proper relationship to the sleeve, expansion of the expandable element of the plunger serves to force the rim portion of the closure into tight engagement with the sleeve all around the same, to effect a sealing bond between the sleeve and the closure. Such a plunger is useful for other purposes when it is desired to provide a temporary radial expansion of a force exerting element, as for example in pressure forming a portion of a tube.
The present invention is of special value for high frequency welding an internal end closure to a tubular container sleeve, especially where the body of the sleeve is of a material such as cardboard or plastic that does not absorb high frequency energy but where there is a layer of material (such as aluminum) that absorbs high frequency energy on the inner surface of the sleeve or the outer surface of the closure rim, or both, and over this layer is another layer of a heat weldable material (such as a suitable plastic) that is heated to its fusion point by induced heating of the energy absorbing layer.
Because of its particular utility in such high frequency bonding, the invention is herein described mainly with reference to the weld sealing of an end closure in a tubular container sleeve.
For maintaining the rim portion of the closure in firm engagement with the tubular sleeve all around the latter during a high frequency welding operation, to produce a seal that is liquid-tight and even gas-tight, it is known to use a radially expandable plunger that is radially expanded when the high frequency current is applied and which is maintained in its expanded condition until the current is cut off and the weldable layers of material have cooled enough to provide a sealed joint. In many cases the closure is formed from an initially flat blank that is flatwise engaged by a bottom surface on the plunger and is forced downward through a forming ring which is fixed just above the container sleeve into which the closure is being inserted. As the plunger forces the blank down through this ring, the plunger and the forming ring cooperate to fold back the rim portion of the closure, and, continuing its downward motion, the plunger carries the closure down into the upper end portion of the tubular sleeve. The plunger that effects this forming and insertion of the end closure is preferably the same one that maintains the rim portion of the closure firmly engaged with the sleeve during the bonding operation.
Expandable plungers heretofore used in operations such as described above are disclosed in U.S. Pat. No. 2,423,965, to Coyle and in the applicant's U.S. Pat. No. 4,640,733 (counterpart of Swedish Patent No. 8302841-5). Each of these plungers comprised two parts, one of which was axially movable relative to the other, and a ring of rubber-like elastic material that was confined between those parts to be axially compressed by their relative motion in one direction and thereby expanded radially outwardly.
The expandable plunger of the Coyle patent provides a very large surface for engagement against the rim portion of the end closure because its expandable element has a substantially large axial extension. It is therefore unsuitable for producing a liquid-proof seal, or even a powder-proof seal, because the material of the expandable element, when expanded, contacts folds and wrinkles in the rim portion of the end closure that are inevitably produced when the end closure blank is forced through the forming ring. Such folds and wrinkles are of increasing size at increasing distances from the flat lid portion of the end closure, and therefore it is desirable to apply radially outward sealing force to the rim portion in a zone that is as close as possible to the flat lid portion, in order to obtain an optimum seal. With the arrangement disclosed by Coyle, the radially outward force applied to the rim portion cannot be a very high one because it is distributed over a large area of the rim portion, and it cannot be exerted in close proximity to the lid portion.
The expandable rubber ring of the plunger disclosed in the applicant's above identified patent has only a small axial extension, so that it applies force to only a limited area of the end closure rim to compress it strongly against the tubular sleeve, and the cooperating parts of the plunger that deform the expandable element are so arranged that its radially expanded portion is close to the bottom surface of the plunger, to exert force on the end closure rim in a zone around it at which folds and wrinkles are at a minimum and thus present the least interference with the production of a tight seal between that rim portion and the container sleeve.
In both of these prior expandable-type plungers the expandable element was substantially in the shape of a ring and was therefore suitable only for containers of circular or substantially circular cross-section. Such rings of rubber or rubberlike material have the further disadvantage that they wear out rather quickly and thus cause shut-downs for their replacement. Such a ring generally cannot be positively guided during its contraction after being expanded, and in a rapidly operating machine it therefore may not fully return to its initial shape before the plunger is fully withdrawn from a container sleeve, with the result that the ring may stick to the rim portion of the closure and be damaged or forced off of the plunger.